1. Field of the Invention
The present invention relates to acetic anhydride, a method of purifying crude acetic anhydride, and a method of producing polyoxytetramethylene glycol using acetic anhydride.
2. Description of Prior Art
Acetic anhydride is widely used as a raw material of cellulose acetate, or in pharmaceuticals, agricultural chemicals (acephate, etc.), dyes, face washing agents, sweeteners (aspartame, etc.), plasticizers (tributyl citrate, etc.), polymer field (polyoxytetramethylene glycol, polyacetal, liquid crystal polymer, etc.), and the like. Particularly, polyoxytetramethylene glycol (hereinafter, abbreviated as PTMG) is an industrially useful polymer which is used as a main raw material of polyurethane, polyether ester and polyether (ester) amide used in spandex, elastomer, artificial leather and the like, or as a surfactant, pressure liquid and the like, and recently, also noticed as an engineering material, medical polymer material and the like typically in the elastomer field.
There are various methods for producing this PTMG, and usually, a method is known in which tetrahydrofuran (hereinafter, abbreviated as THF) is ring-opening-polymerized in the presence of acetic anhydride and a solid acid catalyst to produce polyoxytetramethylene glycol diester (hereinafter, abbreviated as PTMGAC), then, PTMGAC is hydrolyzed in the presence of an alkali catalyst or ester-exchanged with lower alcohol to produce PTMG (e.g., JP-A No. 4-306228).
However, conventional PTMG produced by the above-mentioned method does not necessarily have excellent evaluation results of hue which is an important index of the quality, for example, APHA value, and causes a problem that PTMG which is slightly colored tends to be produced.